1. Field of the Invention
The present invention relates to a wire grid polarizer and, more specifically, to a wire grid polarizer which is preferably used as a polarizing beam splitter for separating linearly polarized lights being orthogonal to each other by transmitting or reflecting the light, in a light pickup optical system, an optical fiber communication optical system, a projection-type liquid crystal projector, a light-receiving sensor optical system and the like.
2. Description of the Related Art
Conventionally, there have been various types of structures known as polarizer which transmits only linearly polarized light with a specific polarization component out of two linearly polarized lights being orthogonal to each other and absorbs or reflects the other polarization component.
Above all, recently, a type of polarizer shown in FIG. 1 as a wire grid polarizer 4 has drawn an attention as the one which exhibits such an excellent property that it can be used not only as a transmission type but also as a reflection type. The wire grid polarizer 4 in FIG. 1 comprises a wire grid structural body 3 in which a plurality of metal wires 2 are arranged at a certain period Λ provided on a light-transmitting substrate 1.
In other words, a conventional polarizer such as a polarizer, which is formed by doping fine metal particles on a substrate and drawing it, transmits only the linearly polarized light of a specific polarization component and absorbs a linearly polarized light of the other polarization component, while the wire grid polarizer 4 has the property of reflecting the linearly polarized light of the polarization component other than the linearly polarized light to be transmitted.
Therefore, not only the linearly polarized light of the polarization component transmitted through the wire grid polarizer 4 but also the linearly polarized light of the reflected polarization component can be picked up to be utilized. Thus, it has been expected to be utilized as a polarizer with a fewer loss in a wide range of optical field such as a pickup optical system, an optical fiber communication optical system, a liquid crystal projector and the like.
In the polarizer, there are two parameters such as a polarization extinction ratio and an insertion loss for indicating the performance.
The polarization extinction ratio is a parameter showing how little a leak of the polarization component is, which is present in the transmitted light and the reflected light, respectively.
When the metal wire 2 is formed and arranged at a certain period in a prescribed direction (in the lateral direction in FIG. 1) as shown in FIG. 1, most of TM polarized light with an electric field oscillation plane being orthogonal to the longitudinal direction of the metal wire 2 transmits through the wire grid polarizer 1 while the majority of TE polarized light with the electric field oscillation plane being in parallel to the longitudinal direction of the metal wire 2 is reflected. The polarization extinction ratios of the transmission and reflection, which indicate the performance of the polarizer 4 at this time, can be expressed by the following expressions:Transmission extinction ratio=−10-Log(Transmission factor of TM polarized light/Transmission factor of TE polarized light)  (1)Reflection extinction ratio=−10-Log(Reflection factor of TE polarized light/Reflection factor of TM polarized light)  (2)
In general, it is considered that a polarizer with a higher polarization extinction ratio exhibits a higher performance.
Parameters which determine the characteristic of the wire grid polarizer 4 are: the type of the metal used for the metal wire 2; the period Λ of forming the metal wire; a duty ratio f (in other words, a filling factor), which is a ratio of a single wire width w to the unit period Λ; and the thickness t of the metal wire.
As the types of the metal to be used for the metal wire 2, it is preferable to use a metal with high reflectance, e.g., gold, silver, aluminum and the like.
In order to achieve still higher performance of the wire grid polarizer 4, it is necessary to prevent a high-order diffraction light from being generated in the wire grid structural body 3. For this, it is necessary that the value of the period Λ satisfies “Λ<λ/2” with respect to the wavelength λ of the incident light. Especially, in order to achieve the high extinction ratio in the broadband wavelength region, it is preferable that the period Λ be further smaller. For example, it is preferable to satisfy “Λ<λ/10”.
In order to have only a zero-order diffraction light, that is, the straight light, it is necessary to satisfy the following expression provided that the angle of incident light which makes incidence to the wire grid polarizer 4 is θ1, the angle of refraction is θ2, the refractive index of the light-transmitting substrate 1 is n2.λ/(n2 sin θ2 −sin θ1)≧Λ  (3)
Further, considering the case of normal incidence and the minimum wavelength of the light to be used is about 450 nm and the refractive index of the light-transmitting substrate is about 1.5 to 2.2, it is predicted that the value of Λ is required to be in the region of about 200 to 300 nm. Needless to say, the value of Λ may be smaller than this.
In general, it is known that the optical property of the diffraction grating with a grating period of less than the wavelength of the light can be estimated precisely to some extent by a calculation according to RCWA method (Rigorous Coupled-Wave Analysis). As for a method utilizing the RCWA method, an example may be a software called G-Solver and the like, which is distributed by Grating Solver Development Company, U.S.A.
The above-described wire grid polarizer is nothing but an optical element in which the period of the metal wires 2 arranged in grid form is less than the wavelength to be used. Thus, it is possible to determine the optical property using the RCWA method.
The results of the diffraction efficiency of the transmission and reflection of TE and TM polarized lights calculated by using the RCWA method are shown in FIG. 2 and FIG. 3 as a transmission factor and a reflection factor.
FIG. 2 and FIG. 3 show the results of the calculation provided that the period Λ of the first-order metal wire grid is 200 nm, the duty ratio is 0.4, the material of the metal wire 2 is gold, and the depth of the grid, that is, the thickness t of the metal wire 2 is used as a parameter.
As can be seen from the transmission factor and the reflection factor shown in FIG. 2 and FIG. 3, the incident light of the TE polarization is mostly reflected at a certain grating depth or more. However, as for the TM polarized light, the transmission factor changes periodically when the grating depth changes.
Next, FIG. 4 is a graph showing the polarization extinction ratios of the transmission and the reflection calculated based on the transmission factor and the reflection factor shown in FIG. 2, FIG. 3 and the expressions (1), (2).
As can be seen from FIG. 4, in order to increase the polarization extinction ratio of the transmitted light, the depth of the grating may be simply deepened.
In the meantime, in order to increase the polarization extinction ratio of the reflected light to the maximum, it is necessary to select the grating depth at which the transmission factor of the TM polarized light becomes the maximum, that is, a specific grating depth so that the reflection factor becomes the maximum.
FIG. 5 shows the wave dependency of the transmission factor and the reflection factor in the band of 0.4 to 0.9 μm and when the initial grating depth at which the transmission factor of the TM polarized light becomes the maximum is set to be 200 nm.
As can be seen from FIG. 5, the extinction ratios of the transmitted light and the reflected light are remarkably decreased especially in the low wavelength region.
There are some reasons which cause the deterioration of the extinction ratio in such low wavelength. One of the known reasons is a phenomenon called anomaly and another is an interference effect of the thin film.
In order to decrease such deterioration, one of the methods is to reduce the grating period, that is, the period Λ of the wire grid to be about the wavelength λ/10. However, it is quite difficult at this point to achieve the processing of such precision even in the optical region of 450 to 650 nm in terms of the resolution of the lithography.
For example, as in U.S. Pat. No. 6,122,103, proposed is a wire grid polarizer in which the polarization extinction ratio in the broadband is increased and leveled by decreasing the effective refractive index of a glass substrate through forming a groove on the glass substrate of the wire grid polarizer.
As a polarizer which achieves high polarization extinction ratio of especially the reflected light in the broadband by eliminating the anomaly in the low wavelength, proposed is a polarizer as disclosed, for example, in Japanese Patent Unexamined Publication No. 2002-328234, in which the polarizer comprises a multilayered-film metal wire made of a dielectric material and a multilayered film of a metal.
However, the wire grid polarizer disclosed in the U.S. Pat. No. 6,122,103 has such a problem that it takes time to process the minute groove formed on the glass substrate.
Also, as for the polarizer disclosed in Japanese Patent Unexamined Publication No. 2002-328234, it has such a problem that it takes time to form and process the multilayered film since it is necessary to form the dielectric material and the multilayered film of the metal using vacuum evaporation and the like and then process the multilayered film to a wire by a pattern of some hundreds nm.
Further, there is a problem in the polarizer disclosed in Japanese Patent Unexamined Publication No. 2002-328234 that it is difficult to perform etching since the types of gas suitable for dry etching used for the metal layer and an oxide layer are different from each other at the time of performing micro-processing onto the grating of the alternate layers of the metal and the oxide.
Thus, as for a wire grid polarizer which enables to maintain the polarization extinction ratio high and level in the broadband wavelength region with a simple structure, an effective proposal has not yet been presented.
The present invention has been designed to overcome the foregoing problems. It is an object of the present invention to provide a wire grid polarizer with an excellent optical property, which enables to maintain the polarization extinction ratio high and level in the broadband wavelength region with a simple structure.